The present invention relates to a mass spectrometer, and more particularly to a mass spectrometer which is provided with an ion detector capable of detecting both a positive ion and a negative ion at high sensitivity.
A conventional ion detector included in mass spectrometers for detecting positive and negative ions is made up of an ion-electron converter, an electron-photon converter, and a photo-multiplier, as described in, for example, an article by H. Tamura et al. ("Shinku", Vol. 19, No. 8, 1976, pages 280 to 288).
The above ion detector can detect both a positive ion and a negative ion, but cannot avoid the generation of noise in the photo-multiplier. Accordingly, in a case where a positive ion is detected, the ion detector is inferior in detection sensitivity to the following ion detector capable of detecting only a positive ion.
Usually, a positive ion generated in a mass spectrometer is detected by an ion detector having the structure shown in FIG. 6A. FIG. 6B shows a potential relation among electrodes shown in FIG. 6A. Referring to FIG. 6A, positive ions which emerge from a mass separator 3 and have a desired mass, impinge on an ion-electron conversion surface 7 (namely, the cathode 7 of an electron-multiplier 8) applied with a large negative potential, to generate secondary electrons. The secondary electrons are multiplied by the electron-multiplier 8, and then sent to a data recording unit 19 in the form of a current signal. The electron-multiplier 8 generates extremely low noise, and hence is widely used for detecting and amplifying positive ions generated in mass spectrometers.
The electron-multiplier 8, however, cannot be used for detecting a negative ion for the following reason. In order to multiply the secondary electrons generated at the ion-electron conversion surface, it is necessary to make the potential of the cathode 7 lower than the potential of a current sending portion 9. The mass separator 3 and a slit 4 are applied with a ground potential. Thus, in order for a negative ion passing through the mass separator 3 to generate a secondary electron at the cathode 7, it is necessary to apply a large positive potential to the cathode 7, as shown in 6B. Since the current sending portion 9 (that is, the anode of the electron-multiplier 8) is applied with a potential higher than the potential of the cathode 7, the data recording unit 19 is obliged to be applied with a large positive potential. In order to solve this problem, a pulse count method is devised in which the direct connection of the anode 9 and the data recording unit 19 is avoided. The pulse count method, however, has the following disadvantage. When the ion optical system of an ion source 2 and the ion optical system between the ion source 2 and the electron-multiplier 8 are improved to increase ions capable of reaching the cathode 7, thereby enhancing ion detection sensitivity, it becomes impossible to detect all ions completely because of short pulse intervals. For example, a mass spectrometer capable of ionizing atoms and molecules under atmospheric pressure is a high-sensitivity analytical instrument, and is used for ultra trace detection. In order to determine ultra trace components, it is necessary to detect small peaks. According to the pulse count method, it is necessary to detect a main peak corresponding to a main component together with the small peaks. When the above ion optical systems are improved so as to increase ions capable of reaching the electron-multiplier 8, an ion current corresponding to the main component becomes greater than 10.sup.-10 A. Such a large ion current cannot be measured by the pulse count method.
In view of the above-mentioned facts, an ion detector with the structure shown in FIG. 7A has been used for detecting a negative ion. Referring to FIG. 7A, a negative ion is converted into an electron by an ion-electron converter 10 which is applied with a large positive potential, as indicated by a dotted line in FIG. 7B. The electron thus obtained is converted into a photon by an electron-photon converter 13 which is applied with a positive potential larger than the positive potential of the ion-electron converter 10. The photon from the electron-photon converter 13 is detected and amplified by a photo-multiplier 15, the output current of which is supplied to the data recording unit 19. The current sending portion 17 of the photo-multiplier 15 is applied with a ground potential. Thus, the data recording unit 19 can be applied with the ground potential.
When the ion-electron converter 10 and the electron-photon converter 13 are applied with a large negative potential and a large positive potential, respectively, as indicated by a solid line in FIG. 7B, the ion detector of FIG. 7A can detect a positive ion. That is, this ion detector can detect both a negative ion and a positive ion.
The ion detector of FIG. 7A, however, has the following drawback. The photo-multiplier 15 is more readily affected by stray light, cosmic rays and others than the electron-multiplier 8 of FIG. 6A, that is, noise is readily generated in the photo-multiplier 15. Hence, the ion detector of FIG. 7A is inferior in signal-to-noise ratio to the positive ion detector of FIG. 6A, and thus cannot detect trace ions.
In order to detect a negative ion by the ion detector of FIG. 7A after a positive ion has been detected by the ion detector of FIG. 6A, it is required to replace the ion detector of FIG. 6A by the ion detector of FIG. 7A. Further, in order to detect a positive ion at high sensitivity by the ion detector of FIG. 6A after a negative ion has been detected by the ion detector of FIG. 7A, it is required to replace the ion detector of FIG. 7A by the ion detector of FIG. 6A. The substitution of one of the ion detectors of FIG. 6A and 7A for the other ion detector is cumbersome, and requires a long time. Hence, it is practically impossible to carry out the above substitution frequently.